A) Field of the Invention
The present invention relates to an open loop fiber optic gyroscope and more particularly to an open loop fiber optic gyroscope which can measure ultra-high rates of rotation.
B) Background of the Invention
FIG. 1 shows a conventional open loop fiber optic gyroscope 1. The gyroscope 1 includes a light source 3, a bidirectional coupler 5, a splitter 7, a modulator 9, a fiber optic sense coil 11, a detector 13, a signal processor 15 and a phase modulation drive circuit 27.
Operationally, light from the light source 3 passes through the bidirectional coupler 5 to the splitter 7. At the splitter 7, the light is split into two beams 8 and 10. The beam 10 passes through the modulator 9 and travels in a counterclockwise direction through the fiber optic sense coil 11. The light beam 8 travels in the clockwise direction through the fiber optic sense coil 11 and then passes through the modulator 9.
After passing through the fiber optic coil 11, as is known in the art, the two beams 8 and 10 are recombined by the splitter 7 and then travel back through the bidirectional coupler 5 and to the detector 13. The output of the detector 13 enters signal processor 15, first being amplified by preamplifier 17 and then converted to digital format by the A/D converter 19.
The output of the A/D converter 19 is demodulated by demodulator 21 and then averaged by signal processor 23. The output of the signal processor 23 is then forwarded output processor 25. It is the signal processor 15 which, in accordance with the techniques described below, calculates the rotation rate of the gyroscope.
The output of the detector 13 represents the intensity of beams 8 and 10 after being recombined by the splitter 7. The signal processor 15 compares the measured intensity of the recombined beams to an interference curve, which is illustrated in FIG. 2, to determine the phase difference between the beams.
As is known in the art, the phase difference between the beams 8 and 10 is proportional to the rotation rate of the gyroscope 1. Thus, if the phase difference between beams 8 and 10 is known, the rotation rate of the gyroscope 1 can be computed.
Referring to FIG. 2, when the rotation rate of the gyroscope is zero, the output of the detector 13 rests at point X on the curve which means that no phase shift exists. When a rotation rate is applied to the gyroscope 1, the output of the detector 13 moves in one direction or the other depending on the direction in which the gyroscope is rotating. For example, if the gyroscope 1 is rotating in the clockwise direction, the output of the detector shifts in a direction toward point A on the curve. Alternatively, if the gyroscope 1 is rotating in the counterclockwise direction, the output of the detector shifts in a direction toward point B on the curve. This phenomenon is referred to in the art as either a rate induced phase shift or Sagnac phase shift.
Since the slope of the interference curve is relatively flat at point X, the sensitivity of the curve at this region to changes in phase shift is low. As such, small directional movements of the rotational rate along this region of the interference curve can not be accurately measured.
To improve the sensitivity and provide a technique for detecting small changes in phase shift, the phases of the beams 8 and 10 of gyroscope 1 are shifted to allow measurements to occur at points A and B on the interference curve. This technique is referred to in the art as phase modulation.
Phase modulation is accomplished by modulating the phase of beams 8 and 10 to allow measurements of the recombined beam to occur at points A and B. The modulator 9 and phase modulation drive circuit 27 shown in FIG. 1 are the devices which perform the phase modulation.
The phase modulation drive circuit 27 contains a square wave generator 31 and amplifier 29. The generator 31 produces a square wave signal which directs the amplifier 29 to apply signals A and B to the modulator 9. When signal A is applied to the modulator 9, the phases of beams 8 and 10 which pass through the modulator, are adjusted such that the output of the detector 13 is measured at or near the region A of the interference curve. Similarly, when signal B is applied to the modulator 9, the phases of beams 8 and 10 which pass through the modulator, are adjusted such that the output of the detector 13 is measured at or near region B of the interference curve.
As is known in the art, the measurement obtained at or near region B of the curve is then subtracted from the measurement obtained near region A of the curve. This calculated difference is then used to determine changes in rotational rate of the gyroscope 1.
While the conventional gyroscope described above allows for measurements to occur at the more sensitive points on the interference curve, it still has significant drawbacks. In particular, the conventional gyroscope does not have the capability of accurately measuring high rates of rotation which induce Sagnac phase shifts in the range of 90.degree..
For example, if the conventional gyroscope 1 were to rotate in the clockwise direction at a very high speed and in turn induce a Sagnac shift in the range of 90.degree., the output of the detector 13 would shift in a direction toward point D on the interference curve given the enormity of the rate induced phase shift. When this occurs, the signal processor 15 cannot distinguish between rotations occurring at points B and D and, as such produces a false reading. Similar errors occur if the output of the detector occurs at point C on the interference curve.
In view of the foregoing, if the conventional gyroscope is placed on an object which at points during its flight has speeds of rotation which induce Sagnac phase shifts in the range of 90.degree., the gyroscope 100 will produce false readings. In view of this problem, there currently exists a need for a gyroscope that can measure ultra-high rates of rotation.